Origins

Lecture 10; May 1 2014

Previously… on Origins:Is Earth a special/unique place?

• What does the question mean?

• How do we find planets?

• What are habitable planets?– Selection effects

Outline: Is Earth a special/unique place?

• Drake’s equation• Is Earth special?

– Heliocentrism vs. Geocentrism– How about in our Galaxy?

• Is our Galaxy special?• Is there any special place?

– Observations

• The cosmological principle and its consequences:– Cosmic Time

A physicist’s answer

• We phrase the question in statistical terms:– Is the location of the Earth in the Universe special

in anyway? – Is it at the “center” of something?

• Let’s take a look! Let’s measure!

Modern “Copernicanism”:Current solar system description

• Newton:– The Sun and the other

bodies orbit around a common center of mass

– The Sun is so massive that it is very close to the center of mass and moves very little

– Orbits are elliptical, but very slightly so

Modern “Copernicanism”:Current solar system description

• Einstein:– Mass perturbs local geometry– Space time is curved– Objects move freely in curved

space time

43s per century!Another test of general relativity!

Modern “Copernicanism”:Parallax and distances

• Measurements of stellar parallax indicate that the closest stars are a few light years away! How far is the sun?

• They also indicate that the Earth “moves” (eppur si muove). With respect to what? How fast?

Modern “Copernicanism”:planets, Sun and other stars

Modern “Copernicanism”:planets, Sun and other stars

Modern “Copernicanism”:planets, Sun and other stars

Modern “Copernicanism”:Where are we in our Galaxy?

Modern “Copernicanism”:Current view

Modern “Copernicanism”:Where are we in our Galaxy?

• Somewhat in the outskirts…

• 25,000 ly away from the center

• Moving at about 200 km/s around the center of the Milky Way

• TRUMPLER’s (1930) discovery of dust

Modern “Copernicanism”:Evidence

The Galaxy is more than a 100,000 lyrs in diameter There are 10,000,000,000 solar masses inside the Sun’s orbit!

Modern “Copernicanism”:Galaxies

What are they? How far are they? How big are they?

Modern “Copernicanism”:Galaxies

• Until 1923 there was a debate on the distance of “nebulae” (galaxies)

• Are they small objects inside our galaxy or are they “external”?

• Hubble settled this by measuring the distance to Andromeda– A whopping 2.5 million

light years!

Modern “Copernicanism”:The Universe is full of galaxies!

10,000 galaxies in a tiny piece of sky! 1/150,000 of the sky

Modern “Copernicanism”:How many galaxies?

Based on the deep fields we estimate of order a billion visible galaxies

Modern “Copernicanism”:Large scale structures

SDSS and 2dF mapped the positions of about 1,000,000 galaxies

Billions of light years

COSMIC MICROWAVE BACKGROUND

Modern “Copernicanism”:Summary

• Our planet orbits around an average star in the outskirts of an average galaxy

• All directions in the universe look the same• All places in the universe look the same if you

average over large enough volumes (100,000,000 light years or so)

• THE UNIVERSE IS HOMOGENEOUS AND ISOTROPIC (Cosmological Principle)

• This scientific hypothesis build on observational evidence allows us to construct a simple theory of the universe, including define a cosmic time

Concluding remarks

The End

See you on Tuesday